1. Field of the Invention
The present invention relates to a touch panel system provided with a projection type touch panel system using an electrostatic capacitance system, an electronic information device provided with the touch panel system, and a method for detecting an indicator position by use of the touch panel.
2. Description of the Related Art
In recent years, a touch panel system for detecting a position of an indicator (for example, a finger of a user, or a touch pen; the same shall apply hereinafter) that is in contact with or close to a detecting surface of a touch panel to accept an instruction of the user is often provided on an electronic information device such as a portable telephone or a display apparatus. Especially, a projection type touch panel using an electrostatic capacitance system, which enables a multi-touch, has often been mounted on an electronic information device.
The projection type touch panel system using the electrostatic capacitance system includes a touch panel having a plurality of drive lines provided along a detecting surface so as to be parallel to each other, and a plurality of sense lines provided along the detecting surface so as to be parallel to each other and so as to cross the drive lines. In the touch panel system described above, when an electric signal (hereinafter referred to as a drive signal) is applied to the drive lines, an electric signal (hereinafter referred to as a sense signal) according to a capacity formed by the drive lines and the sense lines is generated on the sense lines. When this sense lines are acquired and processed, an in-plane distribution of the capacity can be obtained. A region where the capacitance is reduced due to an indicator being in contact with or close to the detecting surface is detected from the obtained in-plane distribution of the capacitance. The position of the indicator that is in contact with or close to the detecting surface is detected based on this region.
In the touch panel system described above, even if there are a plurality of indicators on the detecting surface, the position of each indicator can be detected only by detecting the region where the capacitance is reduced based on the obtained in-plane distribution of the capacitance. However, when a finger is in contact with the detecting surface, for example, noise might be caused along the sense lines on the region where the finger is in contact, in the obtained in-plane distribution of the capacitance, resulting in that a detection accuracy of the indicator might be deteriorated. This becomes a problem.
When a plurality of users simultaneously operate the touch panel (especially, when a plurality of users operate the touch panel by using different indicators), such as when a touch panel system is applied to a large-sized display device, for example, the problem caused by the deterioration in the detection accuracy of the indicator due to the noise becomes noticeable.
The specific example of this problem will be described with reference to the drawings. FIG. 11 is a graph illustrating an in-plane distribution of a capacitance obtained when a finger is in contact with a detecting surface in a conventional touch panel system. FIG. 12 is a graph illustrating an in-plane distribution of a capacitance obtained when a finger and a touch pen are in contact with the detecting surface in the conventional touch panel system. The lower graph in FIG. 11 illustrates the enlarged upper graph in FIG. 11. Similarly, the lower graph in FIG. 12 illustrates the enlarged upper graph in FIG. 12. The graphs in FIGS. 11 and 12 illustrate that a capacitance of a region where no object is present on the detecting surface is defined as a reference, and a region having a capacitance smaller than the reference capacitance projects upward.
As illustrated in FIG. 11, when a finger is in contact with the detecting surface, the capacitance of the region where the finger is in contact is decreased. However, noise is caused along the sense lines passing through the region where the finger is in contact in the obtained in-plane distribution of the capacitance.
In FIG. 11, the noise is sufficiently smaller than the decrease amount of the capacitance of the region where the finger is in contact. Therefore, in FIG. 11, the position where the finger is in contact can be detected in distinction from the noise.
On the other hand, as illustrated in FIG. 12, the decrease amount of the capacitance in the region where the touch pen is in contact is smaller than the decrease amount of the capacitance in the region where the finger is in contact. The noise can be larger than the decrease amount of the capacitance in the region where the touch pen is in contact. This is because a tip end of the touch pen is made narrow (e.g., about 1 mm to 4 mm) in order to enhance usability of the touch pen. Accordingly, when the touch pen is in contact with the sense lines passing through the region where the finger is in contact as illustrated in FIG. 12, it is difficult to distinguish the decrease amount of the capacitance in the region where the touch pen is in contact from the noise, because the decrease amount of the capacitance is buried in the noise. Therefore, the accuracy in detecting the position where the touch pen is in contact is deteriorated.
Japanese Laid-Open Patent Publication No. 2012-22543 discloses a touch panel system that detects positions of a plurality of indicators (electronic pen or finger) that are in contact with a detecting surface. The touch panel system recognizes an identification signal outputted from the electronic pen, thereby identifying the electronic pen, and detecting the position where this electronic pen is in contact.
The touch panel system described in Japanese Laid-Open Patent Publication No. 2012-22543 can use only a special electronic pen, which provides poor usability. Accordingly, a touch panel system has been desired that can accurately detect a position of an arbitrary indicator, which is not limited to the special indicator described above, in distinction from noise.